Fostering temporal crop diversification to reduce pesticide use

Temporal crop diversification could reduce pesticide use by increasing the proportion of crops with low pesticide use (dilution effects) or enhancing the regulation of pests, weeds and diseases (regulation effects). Here, we use the French National DEPHY Network to compare pesticide use between 16 main crops (dilution effect) and to assess whether temporal crop taxonomic and functional diversification, as implemented in commercial farms specialized in arable field crops, could explain variability in total pesticide use within 16 main crops (regulation effect). The analyses are based on 14,556 crop observations belonging to 1334 contrasted cropping systems spanning the diversity of French climatic regions. We find that cropping systems with high temporal crop diversity generally include crops with low pesticide use. For several crops, total pesticide use is reduced under higher temporal crop functional diversity, temporal crop taxonomic diversity, or both. Higher cover crop frequency increases total pesticide use through an increase in herbicide use. Further studies are required to identify crop sequences that maximize regulation and dilution effects while achieving other facets of cropping system multiperformance.

, which quantifies the number of applications at the full recommended dose) for the main crops in which pesticides are commonly applied.Crops with null to low pesticide use (i.e. less than 0.7 for total pesticide use, less than 0.5 for herbicide, less than 0.1 for fungicide and less than 0.2 for insecticide) were not included in these analyses.Significance of slopes was assessed with a two-tailed Wald test, which tests the coefficients (i.e.slopes of diversity indicators for each crop) against zero.Degrees of freedom (df) are labelled as "Inf" (for infinite) in the case of asymptotic results (that is, when estimates are tested against the standard normal distributionz testsrather than the t distribution).SE: standard error; asymp.LCL: asymptotic lower confidence limit; asymp.UCL: asymptotic upper confidence limit.1).Within each graph, violin plots sharing the same letter are not significantly different at p< 0.05 based on a set of two-tailed Wald tests, which assess whether or not the pairwise differences in means between climatic regions are different from zero.Multiple comparisons were adjusted using the false discovery rate method.3761 cropping system described at a given time point (time point referring to either the two-to-three-year average provided by farmers upon entry in the network or the subsequent annual descriptions) were available for all three charts.Total pesticide use (± standard deviation, SD) averaged 3.7±2.9across all 14456 observations (i.e. one crop of a given cropping system at a given time point) of the 16 main crops of the dataset, and 1.6±1.1 for herbicides, 0.9±1.9 for fungicides, and 0.4±1 for insecticides.Herbicides, fungicides, and insecticides represented, on average (±SD), 49±25, 19±18, and 8.5±16 % of total pesticide use, respectively.Coefficients of variation (CV) highlighted higher variability in fungicide (210%) and insecticide (230%) use than in herbicide use (70%).

Supplementary Figure 3 :
Violin plots highlighting the effect of climatic regions on crop diversity indicators.a: Functional diversity, b: Taxonomic diversity and c: Species diversity (i.e.Functional x Taxonomic diversity).Violin plots were created using the observed data.Points represent estimated means and lines associated 95% Wald confidence intervals (computed using the delta method) obtained from a generalized linear mixed effect model (A: model 1, B: model 2 and C: model 3 Supplementary Table

Figure 7 :
Probability density of crop pesticide use.a: Total pesticide use, b: Herbicide use, c: Fungicide use and d: Insecticide use across the 16 main crops considered in this study.Pesticide use of crops was assessed with the Treatment Frequency Index (TFI) which quantifies the number of applications at the full recommended dose.One data point (N=14456) represents a TFI value for one crop in a given cropping system at a given time point (time point referring to either the two-to-three-year average provided by farmers upon entry in the network or the subsequent annual descriptions).If multiple fields were represented by the same crop in a given cropping system at a given time point TFI values for this crop were averaged across fields.Kernel bandwidth was chosen using Silverman's rule-of-thumb method.Probability density was estimated with gaussian kernel method and describes the probability densities associated with TFI values.Vertical solid lines represent the mean.Vertical dashed lines represent the median.x̄ = mean; SD= standard deviation; IQR= Interquartile range

Table 2 : List of crops present in the dataset (vernacular and latin names) and their membership to different botanical families and sowing periods.
Crops are ranked according to their average proportion across all rotations.

Table 3 :
Effect sizes and significance of slopes of crop diversity indicators (functional and taxonomic) on pesticide use (total and per type: herbicides, fungicides, and insecticides, all assessed with the Treatment Frequency Index (TFI